(1) Field of the Invention
The present invention relates to a field emission emitter to emit electrons by a field effect among electron sources utilized for various display elements, light sources, high speed switching devices, micro sensors, and so on. More particularly, it relates to a method for making a silicon field emission emitter which uses a plating material or a metal silicide on a tip of the emitter to enforce or change characteristics of a tip of a silicon field emitter.
(2) Description of the Related Art
Recently, attention is concentrated on the substitution of an inefficient thermionic emitter for a high field emission emitter. The emitter is very efficient since an emitter material does not need to be heated. The emitter has been used for scanning sources of an electronic microscope for several years, and the emitter is now being developed as a source for a vacuum microelectron device, a flat panel display, and a high efficiency and frequency vacuum tube.
The field emission emitter may have very high luminous efficiency and luminance by making a point of the field emission material of which a radius is less than about 100 nanometer high-integrated to 10.sup.4 -10.sup.5 Tips/mm.sup.2, and thus is thought as a very suitable display device for the embodiment of wall television sets owing to a low voltage consumption.
An emitter tip has a generally cone-shaped structure, and methods for constructing the emitter tip are classified as four categories as follows.
A first category is a very initial category that the emitter tip is formed by a direct deposition of the material. The embodiment for the first category is mentioned in pages 3504-3505, No. 7, Vol. 39, Journal of Applied Physics, a paper by C. A. Spint, "THIN FILM FIELD EMISSION CATHODE", and a similar process to the above-mentioned one is depicted in a U.S. Pat. No. 3,755,704.
A second category is to use anisotropic etching of a single crystalline material as a silicon disclosed in a U.S. Pat. No. 3,669,241.
A third category is to use isotropic etching which forms the above-mentioned construction. The embodiment for the third category is disclosed in a U.S. Pat. No. 3,998,678.
A forth category is to form the tip by oxidizing the emitter material. The embodiment for the four category is disclosed in a U.S. Pat. No. 3,970,887.
To be brief, as the structure of the typical field emission emitter is shown in FIG. 1, a cone-shaped microtip emitter is made. A reference numeral 11 indicates a silicon substrate doped with impurities of high density and having high conductively rate. Also, the cone shaped emitter 17 is formed within a cavity 15 in an insulating layer 13 on the silicon substrate. In addition, the emitter is encompassed by a control and extract electrode, and by a gate electrode 19.
All the silicon field emission emitters made by the forth Categories have some limitations. Namely, to obtain the field emission of high efficiency, two conditions, for example, a first condition that a component of the emitter tip have a low work function and a second structural condition that a gate aperture be small, should be satisfied. However, the emission efficiency can not be low because the silicon microtip emitters have a lot of factors hindering the first condition--a low melting point, a low electric conductivity and a high work function. In addition, the silicon emitter tip having a sharp point is apt to be worn away, or be broken off because the silicon emitter tip is sensitive to the impurity and a mechanical strength.
In this point of view, as shown in FIG. 2, to strengthen and vary the characteristic of the emitter (to improve the work function), recently, a study on a newly constructed field emission emitter 17 which a refractory metal silicide layer 20 is formed on a upper part of the emitter is being lively made. But, this emitter also has a following problem. Namely, when metallic material for forming the silicide layer is deposited, the metallic material is deposited on a wall of the insulating layer 13 in the cavity 15 as well as on an entire surface of the emitter 17. Thus, in an annealing process for forming the silicide layer that is a thermally-treated compound of the metal and the silicon, the metallic material is easily diffused to the insulating layer, so that it results in reducing an electronic insulation effect, increasing the leaked current and reducing breakdown voltage. (Refer to IVMC 92' paper, "Gated Silicon Field Emitter Tip Technology" by R. B. Marcus et al.)